The objective of the proposed research is to continue the analysis and description of the various relationships that have evolved in the transmission of viruses by arthropod vectors. Emphasis will be placed on the viruses that are known to use vectors as alternate hosts for their multiplication. Electron microscopic studies will be made of the infection of nonvector species by a propagative plant virus, specifically the sowthistle yellow vein virus in the dark strawberry aphid, Chaetosiphon jacobi, and the strawberry crinkle virus in the sowthistle aphid, Hyperomyzus lactucae. Additional experiments will be made on potato leafroll and filaree red leaf viruses to assess the propagative potential of these circulative viruses.